So that begs the question ... on average, does a cop spend more time [in traffic] in low-speed maneuvers or more at "highway speeds"?
That pretty much depends on the department. Highway oriented organizations work highways and surface streets. City and county police motors generally work surface only leaving highways to appropriate departments. There are no doubt exceptions to the rules. But even "highway" cops can benefit from fine motor skills.
What is the major reason for deploying motorcycle cops over automobile use?
Mobility. Motors can get from Point A to Point B faster than a car in almost any instance. They can pick their way through traffic.
Acceleration. They can respond quicker because of their acceleration. This is usually more important than top speed. When possible most motors hand-off pursuits to patrol vehicles.
Cost. Motors are overall cheaper than cars. So where a motor can be used instead of a car money is saved.
This makes them ideal in many departments for traffic enforcement. While many want to see that as a revenue grab it's also a matter of public safety. Some people like to debate this. I do not. There can be instances where revenue takes precedence over safety. I don't mind as long as the rules are known to every one.
You may see many of them with rifles attached these days. This ain't Mayberry anymore. Some departments need mobility and/with firepower. Some patrol cars are now equipped with both shotguns and long guns.
There is a down side. Motor riders are more susceptible to injury and death in the event of a collisions or attacks from suspects. This can mean increased medical costs. Motor riding is a diminishing skill far more than operating a car. Motor offices need regular training and even instructors need to be rectified regularly. States vary on requirements of course.
Some times you need to break the law in order to enforce the law. This is especially true in stop-light enforcement and speed enforcement. This is can be difficult enough with a motor and often impossible with a car.
It's a dangerous job but somebody's gotta do it.